Biochemical analysis of the epithelin receptor.

Epithelin 1 and 2 are cysteine-rich proteins that act as growth modulators of epithelial cells. In this report, we have characterized the epithelins receptors of MDA-MB-468 human breast carcinoma cells using both binding and cross-linking techniques. Equilibrium binding studies of iodinated epithelin 1 indicated that two classes of binding sites are expressed at the surface of breast carcinoma cells. The high affinity sites had a dissociation constant of approximately 2 x 10(-10) M, with approximately 290 receptors/cell. The low affinity sites had a dissociation constant of approximately 10(-8) M, with approximately 32,000 receptors/cell. Binding of iodinated epithelin 1 was specifically inhibited by unlabeled epithelin 1, 2, or 3, but not by other growth factors tested. We also performed competition binding studies of 125I-epithelin 1 to cell surface receptors in the presence of unlabeled epithelin 1, 2, or 3. Binding results analyzed by the method of Scatchard suggested that all three epithelins interact with a same receptor. A 140-145-kDa epithelin 1-binding protein complex was identified by chemical cross-linking of 125I-epithelin 1 to breast cancer cells. Formation of such a complex was prevented by coincubation of 125I-epithelin 1 with an excess of unlabeled epithelin 1, 2, or 3.